Flag with no name, only for environment variables parsing

[WestleyK]

Checklist

• [x] Are you running the latest v2 release? The list of releases is here.
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What problem does this solve?

I would like to use cli.Flag only to parse environment variables, without a flag.

Example:

package main

import (
  "log"
  "os"

  "github.com/urfave/cli/v2"
)

func main() {
  app := &cli.App{
    Flags: []cli.Flag{
      &cli.StringFlag{
        EnvVars: []string{"DEV_VAR"},
      },
      &cli.StringFlag{
        EnvVars: []string{"ANOTHER_DEV_VAR"},
      },
    },
  }

  err := app.Run(os.Args)
  if err != nil {
    log.Fatal(err)
  }
}

Output:

$ ./foo 
foo flag redefined: 
panic: foo flag redefined: 

goroutine 1 [running]:
[...]

Solution description

A detailed description of what you want to happen.

Describe alternatives you've considered

Alternative is to set the flag as hidden, but I dont want a flag at all.

[rliebz]

AFAIK what you want to do isn't possible—there a lot of assumptions in the code base about flags being command-line flags, and those are unlikely to change. For example, the primary way of accessing a variable through a *cli.Context is through it's name, which is the same as the name of the CLI flag.

I'd personally recommend using hidden—you can use a nonsense name so that users are unlikely to conflict with it, but you still have a unique identifier. Type one out, or for a higher level of obscurity, do something like generate a UUID at runtime:

// first library I found on Google, but there are others
import "github.com/google/uuid"

var (
  keyDevVar = uuid.New().String()
  keyAnotherVar = uuid.New().String()
)

// ...

  app := &cli.App{
    Flags: []cli.Flag{
      &cli.StringFlag{
        Name: keyDevVar,
        EnvVars: []string{"DEV_VAR"},
      },
      &cli.StringFlag{
        Name: keyAnotherDevVar,
        EnvVars: []string{"ANOTHER_DEV_VAR"},
      },
    },
  }

If you never need flags and only need env vars, there might be better tools to accomplish what you're looking for. https://github.com/kelseyhightower/envconfig is a wonderful tool that just does env vars, not CLI flags or args.

[WestleyK]

It would be handy if this cli package automatically handles this (either internally generate a uuid, or just dont have any flag). Maybe for v3?

[dearchap]

@WestleyK I agree with @rliebz . You are using the incorrect tools for your use case. envconfig will better serve you well.

[dearchap]

Not a good use case to implement directly in urfave/cli. User could use alternative mechanisms.